It's an aesthetic issue

Light pollution in most areas of the country that are populated takes the night sky away from us. I'm not sure what tangible negative impacts it has, but I do know how nice it is to be able to look up and admire all that stuff above you and how cool it is to be a part of it. I have no idea what kind of technology would be needed to reduce light pollution.

Yes, it is a major threat to numerous species of animals.

The list of different species being adversely affected by light pollution is quite extensive and it affects the populations in various ways. Some species, such as night jars and night hawks have learned that flying bugs flock to lights at night, so the birds are drawn there for the bugs. But many times the light they are flying to are inside buildings and the birds strike the windows and die. Numerous species of nocturnal birds are migratory and lights can confuse them.

Sea turtles survival is also threatened. When the babies hatch, they begin crawling to the light instead of to the sea. When the sun comes up they are easy prey.